<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<h1>0</h1>
<script>
    let count = 0;
    //开启定时器
    let timer = setInterval(f,1000);

    function f() {
        count++;
        let h1 = document.querySelector("h1");
        h1.innerText = count;
        if (count==100){
            //停止定时器
            clearInterval(timer);
        }
    }
    //匿名方法开启定时器
    setInterval(function () {
        console.log(count);
    },1000)

    //开启只执行一次的定时器
    setTimeout(function () {
        alert("时间到！")
    },3000);
</script>
</body>
</html>